CAPÍTULO 3 RESULTADOS
3.1 Antecedentes de la unidad de análisis o población
The theoretically predicted distributions of signal and background are produced through the use of numerous Monte Carlo event generators. While major back- ground contributions such asZ →τ τ and fake taus are modelled using data-driven techniques, their development and validation still depend on simulation. Smaller background contributions are estimated directly from the simulated samples. The simulation of VBF signal is a hybrid procedure with Monte Carlo generation of an SM sample and re-weighting of events from this sample to describe non-zero CP- odd mixing parameter values. This section will describe the MC generation, while section 4.3 details the re-weighting technique. All Higgs samples are produced with a Higgs mass ofmH = 125 GeV. Table 4.1 shows the complete list of MC simulated
samples used in the analysis as well as the perturbative order of the QCD calculation used for each sample.
4.2.1 Signal Samples
Although several Higgs production modes are included in the analysis, only VBF H → τ τ and H → W W production modes are considered to be signal, since the goal is to test CP-invariance in VBF. Samples of 2×106 parton-level VBF events are generated for both decay modes using Powheg [152]. Simulation of the un- derlying event, parton showering and hadronisation is subsequently performed in
Pythia8 [102] at next-to-leading order in QCD and using the CT10 PDF set [88].
The event selection and background discrimination procedure exploit a large num- ber of kinematic variables, making it important to have high precision modelling of the signal events. An electroweak correction of the Higgs transverse momentumpH
t
distribution is introduced by comparing the output of Pythia to the distribution obtained fromHawk[79–81], which takes the complete NLO electroweak contribu- tions into account. The correction size depends onpHT, increasing from 1-2% at low values to approximately 20% atpHT = 300 GeV [55].
Signal MC generator σ× B[pb] Order
VBF,H→τ τ Powheg+Pythia8 0.100 (N)NLO [56, 129–131]
VBF,H→W W Same as VBFH→τ τ 0.34 (N)NLO [56, 129–131] Background
ggF,H→τ τ Minlo+Pythia8 1.22 NNLO+NNLL [56, 132–137]
ggF,H→W W Powheg+Pythia8 4.16 NNLO+NNLL [56, 132–137]
W H,H→τ τ Pythia8 0.0445 NNLO [56, 138]
ZH,H→τ τ Pythia8 0.0262 NNLO [56, 138]
W(→lν), (l=e, µ, τ) Alpgen+Pythia8 36800 NNLO [139, 140]
Z/γ∗(→ll),
Alpgen+Herwig 13000 NNLO [139, 140]
10 GeV< mll<60 GeV Z/γ∗(→ll),
Alpgen+Pythia8 3910 NNLO [139, 140]
60 GeV< mll<2 TeV
VBF,Z/γ∗(→ll) Sherpa 1.1 LO [141]
t¯t Powheg+Pythia8 253 NNLO+NNLL [142–146]
Single top: W t Powheg+Pythia8 22 NNLO [147] Single top: s-channel Powheg+Pythia8 5.6 NNLO [148] Single top: t-channel AcerMC+Pythia6 87.8 NNLO [149]
qq¯→W W Alpgen+Herwig 54 NLO [150]
gg→W W gg2WW+Herwig 1.4 NLO [151]
W Z,ZZ Herwig 30 NLO [150]
Table 4.1: Signal and background samples used in the analysis and the Monte Carlo generators used to model them. All Higgs samples are generated with mass
mH = 125GeV. The cross sections times branching fractions (σ× B) are quoted for
√
s = 8 TeV alongside the perturbative order of the QCD calculation. The signal
processes include the SM H → τ τ and H → W W branching fractions, and the
W andZ/γ∗ backgrounds include the leptonic decay branching fractions. Inclusive
cross sections are quoted for all other backgrounds.
4.2.2 Background Samples
Background samples include all Higgs production modes except VBF in addition to all non-Higgs processes. For gluon fusion Higgs production, ggF, generated with standardPowheg(the generator for this process in the couplings analysis) a large discrepancy is observed between Bjorkenxvalues of the initial state partons at gen- erator level and when calculated at reconstruction level. This ggFPowhegsample is only NLO for 0 jets, which means that only one parton comes from the hard interaction, while any additional jets will originate from the parton shower. This makes the sample unsuitable for calculating the Optimal Observable, since it makes use of both the leading and sub-leading jet in the event. In the simulated VBF signal events these jets both need to originate from the hard interaction in order to have an accurate value for the matrix element used as input to the Optimal Ob- servable calculation. In this analysis the Higgs-plus-one-jet process is simulated at NLO accuracy in QCD withPowhegusing theMinlofeature [153]. ThePowheg
event generator is interfaced to Pythia8, and the CT10 PDF set is used. Asso- ciated V H production is simulated using Pythia8 using the Cteq6l1 [89] PDF set. Contributions from associatedttH¯ production were previously evaluated in the H→ τ τ couplings analysis [2] to be negligible and are not included. As with VBF production, the background Higgs production modes are simulated with H → τ τ and H→ W W decays since the contribution from H →W W inτ`τ` is potentially
non-negligible in the signal region of the analysis.
Other background samples are generated using various event generators in- terfaced to eitherPythiaorHerwig[103] to simulate the underlying event, parton shower and hadronisation. For theHerwigsamples the tau lepton decays are simu- lated usingTauola [154]. Photon radiation from charged leptons in these samples is calculated byPhotos[155]. Samples containingZ/γ∗ + jets andW + jets events are generated with Alpgen [156] by using the LO matrix elements for W and Z production including a maximum of five additional partons in the final state. It uses a matching scheme [157] between the matrix element and the parton shower algorithm. Z/γ∗ + jets events are generated in two separate intervals of the true dilepton mass with mtruell ≶ 40 GeV. This was done to avoid generating excessive events with a dilepton mass below the event selection requirements of the analysis. The low-mass samples are interfaced toHerwigandJimmy [158], while high-mass samples are interfaced toPythia. SinceZ/γ∗+ jets events constitute a considerable fraction of the background in signal enriched kinematic regions, a high statistical power is desirable when optimising methods of background rejection. Therefore, additional VBF-enriched samples were generated by applying a VBF-like kinematic filter at generator level before detector simulation that allows the simulation of large samples of events in the kinematic region sensitive to signal.
Event samples with top quarks are generated separately depending on the process and channel. At¯t sample is generated usingPowheg [159, 160] interfaced
toPythiausing theCteq6l1PDF set. The s-channel and W tprocesses in single-
top events are likewise generated with Powheg interfaced to Pythia, while the t-channel processes are generated usingAcerMC[161, 162] interfacted toPythia. In the τ`τhad decay channel these samples are only used to model the part of top
background where theτhad object originates from a realτhaddecay or a misidentified
light lepton. Events where jets are faking hadronic tau decays are instead modelled by the data-driven fake factor method described in section 4.5.2.
Production of diboson events is simulated withHerwigin the case ofZZand W Zevents, whereasW W events are simulated withAlpgeninterfaced toHerwig. The loop-inducedgg→W W process is generated using thegg2WW[163] program
interfaced toHerwigandJimmyto model non-perturbative QCD effects. Alpgen has a superior description of the jet topology in diboson events, but off-shell Z contributions are not included inZZ andW Zleading instead to the use of Herwig in these cases.